The basic idea of modification #3 is building a clip that disables the onboard oscillator, and feeds in a new, faster signal. The beauty of this modification over the others is that you do not have to do any soldering on the motherboard itself, just on the part you clip onto the surface mount crystal oscillator in your Mac. This is the modification most people use on the C610, C650, C660av, Q610, Q650, Q660av, Q800, Q840av, PM6100/60(av), PM6100/66(av), PM7100/66(av), PM7100/80(av), PM8100/80(av), and PM8100/100(av). It will only work on machines with surface mount crystal oscillators.
The really neat thing about this came into play in February 1992 when Apple released the Centris 610, 650, and Quadra 800. In these machines and since, Apple has been using surface mount crystal oscillators. Now that Apple was using surface mount crystal oscillators, there was plenty of accessible area on the metal tabs of the oscillator. In June '93 Guy Kuo reported the first crystal swap of sorts on a Centris 610 to the net. He soldered pins 3, 5, 10, and 12 of a 14 pin socket directly onto the surface mount crystal oscillator. Because the pins on a standard 14 pin package TTL crystal oscillator are at positions 1, 7, 8, and 14, he made jumpers between pins 5-7, 8-10, and 12-14. He disabled the on-board surface mount crystal oscillator with a jumper between 3-5. Then put the new crystal in the socket. The complete text of his work is available as centris-610-clock-mod-11.txt.
I was a little hesitant about soldering onto my new Quadra 800, so wrote to him a few days later about using a 3M Surface Mount Test Clip, and asked his thoughts. He suspected I could not find a reasonable test clip, but otherwise agreed it would work. A few days later the 3M SOIC test clip arrived, and the test clip worked perfectly. I was running my Quadra 800 at 40MHz with no problems, and best of all the modification was all contained in a simple little clip that could be removed without trace at will. And thus the removable test clip approach was born. My Q800 even worked at 48MHz as long as I did not access the serial ports. A few days later I got several crystals, and found the highest frequency on my Quadra 800 to be 42.0MHz. Since then I've tried it at 42.1052MHz, and the serial ports did not work, so the cutoff for my Q800 was at 42.0MHz. If you never use your serial ports, 48MHz worked fine for me, while at 50MHz my Mac was not happy and would not boot.
So if you are still interested, you will need a surface mount test clip; 3M and Pomona make them, and I prefer the 3M ones. Make sure you get a surface mount test clip. The I.C. test clips also work, but I prefer the surface mount SOIC (small outline integrated circuit) ones. A 10, 12, 14, 16, or 18 pin clip will be fine. I'd say go with a 14 or 16 pin narrow or wide clip. I used to recommend the gold coated ones, but the resistance/corrosion effect is minimal over the alloy ones. You will also need a 14 pin IC socket, there are plenty of types. The machined pin ones are nice because you can pop out the pins that are not needed to get them out of the way since you only need three pins in the socket. (Yes, just three, pin one on the new oscillator is not going to be used) You will also need an oscillator (more on this later), a little wire, soldering iron, solder, and possibly a heat sink and or fan depending on the machine. For a C610, C660av, Q610, and Q660av you should add a heat sink. And you will want a fan with the PM's.
The others already have heat sinks, and do not get too hot. I had an extra fan with my Q800, but removed it, and it has been fine. The heat sinks come with the clips needed to attach them to the chip. These are a bit of a pain, you just have to work at it for a while. There may be several ways to do it, but I just slide the clips on from the side. Sometimes they fall off half way there, but eventually it works. Some people have been using the heat sink/fan combo's.
The new Q610 and Q660av computers are based on a new mask of the 68040 (There is an "H" after the '040 and before the "RC") that runs cooler at 25MHz, so it comes at 25MHz without a heatsink. This is the same mask as the C660av and Q840av uses. If you do the modification on them it would still be best to add a heat sink.
Stand the clip so it's jaws are facing down, and the rows of
pins go from left to right, and call the closer row A and the
further row B. Number the pins from left to right 1 through 7
(for the 14 pin clip). Next place the 14 pin IC socket with the pins
down, and the notch to the left, and number the pins as 1, 2, 3,
4, 5, 6, 7 in the row closest to you, going left to right. The
other row is numbered 8, 9, 10, 11, 12, 13, 14 as you go right
to left (back towards the notch).
There are several surface mount oscillators used on the
motherboards. The proper surface mount crystal oscillator on
the mother board will have a frequency on it half that of your
computer and can be determined from the Machine Specifics Page.
That is it, now you just clamp it onto the surface mount
crystal oscillator with the notch on the socket facing the same
way as the surface mount crystal oscillator. And watch to
make sure the little pins clamp onto the surface mount chip.
You may want to use a flashlight for this. These clips hang on
very, very well, I've never had mine move in the last year and a
half, nor any of the other ones I've done.
Now just turn on your computer and enjoy the speed.
Now starting with the test clip,
remove all the pins but four, leaving pins in positions A2, A6,
B2, and B6. Next solder a little jumper wire between pins A2
and A6. Now get the 14 pin IC
socket, and remove all the pins but 7, 8, and 14. Solder a
jumper wire from pin 7 on the IC socket to the jumpered pins
on the clip, either A2 or A6 will do. Also solder a jumper wire
from pin 8 to pin B6, and pin 14 to pin B2.
If you get the narrow clip, you may want to replace the spring
with one with less tension; they are like $0.30 at hardware
stores, and I cut them into two springs. This way you don't
have to push so hard, and it is easier to position on the
motherboard. Now put the crystal in the socket with pin 1 in
1, 2 in 2, 3 in 3 and 4 in 4.
Marc Schrier
(schrier@mac.com)